The general outlines of the mechanisms by which external stimuli effect the behavior of target cells have been described in general Molecular Biology textbooks over the last 10-20 years. For at least some of these stimuli, a primary interaction of the stimulating agent at a cell surface receptor is translated into an effect on various secondary signaling pathways internal to the cell, which secondary signaling pathways in turn produce the observed effect on cellular behavior. Most of these secondary pathways involve the synthesis and hydrolysis of phosphorylated acyl glycerol derivatives such as phosphatidic acid, phosphatidyl inositol, phosphatidyl ethanolamine, lysophosphatidic acid, and so forth. The synthesis and release of the components of these compounds can result in cellular proliferation, suppression of proliferation, differentiation, activation, and so forth, depending upon the nature of the target cell and the stimulus applied.
The pathways regulating the synthesis and degradation of phosphorylated derivatives of acyl glycerols are complex and interlocking. Certain effects of external stimuli are seen immediately--i.e., within a few seconds or a minute; others are seen 30-60 minutes after the external stimulus has bound to the surface receptor of the cell. It is believed that the short-term effects on these second messengers are associated with the stimulus itself and are not appreciably interconnected with those aspects of the phosphorylated acyl glycerol (PAG) pathways that regulate normal cellular processes.
As demonstrated hereinbelow, a short-term effect of a primary stimulus on a target cell is to elevate the levels of specific unsaturated subspecies of phosphatidic acid (PA) and the corresponding diacylglycerol (DAG) formed by the hydrolysis of this PA. It is known that DAG may be generated by other secondary mechanisms such as the hydrolysis of phosphatidyl inositol (PI) or phosphatidyl ethanolamine (PE). However, the nature of the acyl groups of the DAG derived from these various sources is not identical. In particular, DAG derived from PA hydrolysis has a high level of sn-2 unsaturation not containing arachidonate (C20:4).* Typical fatty acid residues found in these PA/DAG subsets include those of oleic (C18:1), linoleic (C18:2) and docosahexanenoyl (C22:6). FNT * This notation refers to the number of C in the acyl residue (20) and the number of .pi. bonds (4).
Further explanation of the model of cell activation and its relation to the compounds of the invention as found by applicants is set forth hereinbelow.
There are a large number of contexts in which it is desirable to protect target cells from primary stimuli which are the result of, for example, disease states (such as malignancy, autoimmune diseases, or infection) or of medical intervention (such as bone marrow transplantation or chemotherapy) which have negative sequelae in the target cell. This protection can be achieved by the method of the invention.
Some of the compounds useful in the method of the invention have been suggested for medical use in other contexts. Pentoxifylline (1-(5-oxohexyl-3,7-dimethylxanthine) is one member of this class of xanthine derivatives which has seen widespread medical use for the increase of blood flow. Pentoxifylline and its use as a vasodilator are disclosed in U.S. Pat. Nos. 3,422,107 and 3,737,433. The nature of the metabolism of pentoxifylline was summarized by Davis, P. J. et al., Applied Environment Microbiol (1984) 48:327-331. Some of the metabolites are also among the compounds of the invention. The immediate reduction product which is the primary metabolite of pentoxifylline--1-(5-hydroxyhexyl)-3,7-dimethyxanthine--was disclosed to increase cerebral blood flow in U.S. Pat. Nos. 4,515,795 and 4,576,947.
In addition, a number of patents have issued on the use of tertiary alcohol analogs to compounds of this class in enhancing cerebral blood flow. These include U.S. Pat. Nos. 4,833,146 and 5,039,666.
Furthermore, U.S. Pat. No. 4,636,507 describes the ability of pentoxifylline and its primary metabolite to stimulate chemotaxis in polymorphonuclear leukocytes in response to a known stimulator of chemotaxis. The ability of pentoxifylline and related tertiary alcohol substituted xanthines to inhibit the activity of certain cytokines on chemotaxis is disclosed in U.S. Pat. No. 4,965,271 and U.S. Pat. No. 5,096,906. Administration of pentoxifylline and GM-CSF decrease tumor necrosis factor levels in patients undergoing allogeneic bone marrow transplant (Bianco, J. A. et al., Blood (1990) 76:Supplement 1 (133a:522). The reduction in assayable levels of TNF was accompanied by a significant reduction in transplant-related complications. However, in normal volunteers, TNF levels are higher among PTX recipients. It does not, therefore, appear that elevated levels of TNF are the primary cause of such complications.
It has now been found that the compounds described hereinbelow can be used systematically to maintain the homeostasis of a large number of target cells in response to a variety of stimuli. In addition, routes to administer such compounds which permit effective dosages to be provided are disclosed.